The present invention relates, generally, to an overcurrent switch which protects an electric circuit from being damaged by an overcurrent, and more particularly relates to a midget overcurrent relay which is comprises a conventional reed relay.
An overcurrent switch is one of the most important electric parts of an electric circuit. It acts to protect the electric circuit from being damaged by an overcurrent which flows in the electric circuit when a malfunction occurs. In the prior art, there are typically three kinds of overcurrent protective devices. A first type of overcurrent protective device is a fuse which is fusible with heat. When an overcurrent flow through the fuse connected in series with an electric circuit, the fuse disintegrates with heat and, accordingly, stops the current from flowing through the electric circuit. A second type of overcurrent protective device is an electromagnetic relay. It comprises an electromagnetic coil and a mechanical switch which is electromagnetically actuated by a current flowing through the electromagnetic coil. When an overcurrent flows in an electric circuit connected through the mechanical switch and also flow through the electromagnetic coil, the electromagnetic coil actuates the mechanical switch and turns the mechanical switch OFF. The overcurrent then ceases to flow through the electric circuit. A third type of overcurrent protective device is an electronic switch which comprises one or more transistors. When an overcurrent flows through an electric circuit, in which the electronic switch is installed, a voltage exceeding a predetermined voltage level is applied to a base electrode of a first transistor. Then, the first transistor turns ON and turns a second transistor OFF, causing the overcurrent to cease flowing through the electric circuit.
Each of the three types of overcurrent protective devices mentioned above has a well known defect. These defects are as follows.
A fuse, that is the above-mentioned first type of overcurrent protective device, disintegrates with heat once an overcurrent flows therethrough. Accordingly, one fuse can not be used over and over again, and once an overcurrent flows through a fuse it has to be exchanged for a new one. An electromagnetic relay, that is the above-mentioned second type of overcurrent protective device, is relatively big and heavy. An electronic switch, that is the above-mentioned third type of overcurrent protective device, is small and light and, further, can be used over and over again. However, since the electronic switch is comprised of transistors, it is easily damaged by an excessively high current or an excessive over-voltage.
Therefore, it is the principal object of the present invention to provide an overcurrent switch which can be used over and over again, is relatively small, lightweight and is durable with respect to an excessive overcurrent and/or an excessive over-voltage. It is another object of the present invention to provide an overcurrent switch which can be constructed simply by utilizing a conventional reed switch and a few simple components attached thereto; and in which the threshold, at which the flow of overcurrent through the electrical circuit is to be stopped, is easily and freely predetermined by using the overcurrent switch.